This application is related to Japanese Patent Application No. 2001-255131 filed on Aug. 24, 2001, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a stacked photoelectric conversion device including a plurality of photoelectric conversion element layers. The stacked photoelectric conversion device can effectivery utilize light of wavelengths in a greater region, and therefore can suitably be used as a stacked solar cell.
2. Description of Related Art
A silicon solar cell utilizing a crystalline silicon substrate has currently been in a mainstream. However, a thin film solar cell including a thin silicon film deposited on a glass or metal plate is now under development with the aim of reducing the manufacture cost and the usage of silicon materials. On the other hand, from the viewpoint of the device structure, a stacked solar cell is also under development to improve photoelectric conversion efficiency and reduce the cost per power generation output.
The stacked solar cell includes a plurality of photoelectric conversion element layers (pn junction layers, or simply referred to as xe2x80x9cphotoelectric conversion elementsxe2x80x9d) which are stacked and electrically connected in series. In general, semiconductor materials functioning as the photoelectric conversion elements are arranged in descending order of semiconductor band gap (Eg) from the light receiving side. This structure allows the stacked solar cell to absorb light of wider range from short wavelengths to long wavelengths. An output voltage is obtained as the sum of outputs of the photoelectric conversion elements and a current value decreases, so that loss of internal resistance is reduced by a large amount.
In the stacked solar cell, incident light such as solar light passes through the stacked photoelectric conversion elements while being absorbed in increasing order of wavelength. Therefore, structure design and manufacture steps of the elements are greatly complicated as compared with a solar cell of unijunction structure. Especially, since the photoelectric conversion elements are serially connected, electric current generated during the operation of the stacked solar cell is limited by short circuit current density of a part of the photoelectric conversion elements which generates the lowest current. Therefore, it is quite difficult to obtain high photoelectric conversion efficiency.
In this connection, Japanese Unexamined Patent Publication No. Hei 11 (1999)-214728 proposes a stacked photoelectric conversion device in which a textured structure (i.e., an uneven surface) is adopted. More specifically, this is a tandem-type (stacked) thin silicon film photoelectric conversion device comprising a first photoelectric conversion unit made of a thin silicon film containing a microcrystalline silicon substance and a second photoelectric conversion unit made of a thin amorphous silicon germanium film (on the light receiving side), wherein the first photoelectric conversion unit includes a textured structure, i.e., fine unevenness provided on the upper surface thereof. This photoelectric conversion device is intended to both of cost reduction and improvement in performance.
According to this prior art, it may be expected that the fine unevenness on the upper surface of the first photoelectric conversion unit brings an optical confinement effect, i.e., an increase in optical path length in the second photoelectric conversion unit. However, interface reflection cannot be expected because refractive indices of the first and second photoelectric conversion units are very close. Therefore, light incident on and passed through the second photoelectric conversion unit without being absorbed therein is not reflected at the interface, and therefore incident on and absorbed in the first photoelectric conversion unit. This is problematic because the short circuit current density of the stacked solar cell is limited depending upon the light absorption ability of the second photoelectric conversion unit.
Further, Japanese Patent No. 2738557 discloses a multilayered solar cell having an intermediate layer (selective reflection film) inserted between the photoelectric conversion elements. The thickness of the intermediate layer is appropriately selected such that incident light is selectively reflected or passes through in accordance with the wavelengths matching with spectral sensitivities of the photoelectric conversion elements. However, according to this prior art, there is no consideration to the unevenness on the surface of the intermediate layer inserted between the photoelectric conversion layers. Therefore, it is not expected that the short circuit current density increases due to the increase in optical path length in the photoelectric conversion element.
An object of the present invention is to provide a stacked photoelectric conversion device which has electric characteristics improved by increasing external current density and thus is suitably used as a stacked solar cell.
According to the present invention, provided is a stacked photoelectric conversion device comprising at least two photoelectric conversion element layers sandwiched between a first electrode layer and a light receiving second electrode layer, and at least one intermediate layer sandwiched between any two of said at least two photoelectric conversion element layers, wherein the intermediate layer has uneven surfaces on a light receiving side and a light outgoing side, the uneven surface on the latter having a greater average level difference than that on the former.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.